Stainless steels have a wide range of applications due to their high corrosion resistance and strength. However, their machining remains challenging, hence limiting the performance and sustainability of machining operations. Tool wear is one of such key limiting factors, which is attributed to low thermal conductivity, work-hardening tendency and high forces in machining. In this study the performance of a new multilayer TiAlN-TiAlCrN PVD coatings with varying Cr content is compared to the commercial TiAlN reference in longitudinal machining of 316L stainless steel. In general, TiAlN-TiAlCrN coatings have higher crater wear rate than TiAlN, where high-Cr content variants are more prone to cratering. However, the new multilayer coatings have comparable or lower rate of flank and notch wear. Further analysis of crater wear revealed that the high-Cr content coatings exhibit increased oxidation and diffusion wear rates at the tool–chip interface, accelerating coating degradation

Titanium alloys are known for being difficult to machine. Within the groups of Ti alloys, the machinability is reduced when going from the alpha-alloys, to alpha + beta, and finally to near-beta or beta-alloys. Uncoated cemented carbide is traditionally used for machining these alloys and finding a suitable coating to improve the performance is a challenge due to the high strength of Ti alloys and the high chemical reactivity of Ti with tool and coating materials at the temperatures achieved during cutting. The PVD applied TixAl1-xN (x = 0.4-0.7) is generally recommended for machining Ti alloys and a top layer of NbN has shown promising performance in milling applications. This study systematically explores the wear mechanisms of either uncoated WC-12%Co or its Ti0.45Al0.55N-NbN coated version in the milling of Ti alloys ranging from near-alpha (Ti-6Al-2Sn-4Zr-2Mo), alpha + beta (Ti6Al-2Sn-4Zr-6Mo), and near-beta (Ti-5Al-5Mo-5 V-3Cr) alloys. The wear evolution is explored by studying as-worn tools and their cross-section using SEM-XEDS and EBSD after having reached 10%, 30%, 50%, and 100% of the full tool life at a flank wear criterion VB = 200 mu m. The coating is removed within seconds of engagement and fails by cracking within the bulk and PVD droplet defects can initiate such cracks. Diffusional dissolution of the coating may be active on a minor scale, but the rapid mechanical failure shows the need for a more robust coating formulation. Exposed cemented carbide is worn at varying intensities when machining the Ti alloys. The highest wear rate is achieved in milling alpha + beta Ti-6246, followed by a moderate wear rate in near-beta Ti-5553, and with a slower wear rate in milling near-alpha Ti-6242 which is explained varying intensities in oxidation wear, diffusional dissolution, and mechanical cracking. Diffusional loss of C gives rounder WC grains and remaining W at the interface is removed by the chip flow. Diffusional loss of Co gives reduced grain bonding and loss of the dampening effect that leads to fractures in WC grains. Specifically in milling alpha + beta Ti-6246 and in minor scale in near-alpha Ti-6242, there is formation of CoWO4 ceramic present several mu m into the tool and its formation is facilitated by the oxidation of binder with resolved W. Fractures within the CoWO4 explains the high wear rate. Cracks also propagate in binder regions initiated from weakened interface regions due to diffusional loss of C and Co.

Improvements in machinability by alloying of the workpiece often adversely impact the end user properties of a material. For example, the common use of non-metallic inclusions can lead to improved tool life during turning or milling, but often adversely affects weldability, corrosion, and wear resistance. A cutting tool material meets kilometers of workpiece material during a machining operation. Hence elements in small quantities in the workpiece may insignificantly affect the end user properties but may have large effects on tool wear. One such effect is the formation of refractory and wear resistant reaction products between the workpiece and tool. Such reaction products forming on tool surfaces may lead to improved machinability. This paper proposes the use of small amounts of alloying to induce such a Tool Protection Layer. Additionally, the paper develops a computational framework for designed alloying which balances formation of Tool Protection Layers, its in-process retention, and the functional properties of the alloy. The method has been validated for a case of manganese steel. The calculations were validated first by a wide range of diffusion experiments. Then by industrial turning of cast alloys, by comparing one reference and two newly designed alloys based on the alloying concept. The alloy with 0.003 mol fractions of Al resulted in more than 3 times increase in tool life, due to in-operando formation of Al2O3 Tool Protection Layer. The designed manganese steel maintained its functional properties with respect to abrasive wear resistance and retained its ability to work harden.

Polycrystalline cubic boron nitride (pcBN) is a very promising tool material for turning martensitic stainless steels at high cutting speeds (vc > 200 m/min). The competitive advantage of pcBN over cemented carbide increases as the cutting speed is increased. Changing the speed might lead to a shift in the wear balance and hence the knowledge about tool wear below vc = 200 m/min might not be applicable at vc = 600 m/min. The coatings designed for the lower speed range might also not be performing in the same way at higher speeds. This paper investigates the wear mechanism of uncoated and (Ti,Al)N coated pcBN tools when turning 17–4 PH in a hardened condition at speeds vc = 200–600 m/min. Both scanning and transmission electron microscopy are used to study the worn tools. The in-depth analysis reveals that adhesive wear is only active at low speeds. Increasing the speed does however lead to more wear by diffusion and oxidation. The cBN is preferentially worn out, leaving the TiC binder at the tool-chip interface. Oxidation results in the accelerated wear of the pcBN but also in the formation of metal oxides within the adhered build up layer. The (Ti,Al)N coating does not significantly extend the tool life within this speed range, but it suppresses the adhesive wear mechanism preventing premature tool failure.

Conventional cemented carbide is recommended for machining Ti6Al4V. However, polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (pcBN) also show promise. Demands for higher productivity accelerate diffusional dissolution and chemically driven wear mechanisms in these tool materials. This study investigates active wear mechanisms by studying the interactions between Ti6Al4V and PCD, pcBN, and cemented carbide tools in diffusion couples at temperatures from 900 degrees to 1300 degrees C. All tool materials suffered from diffusion to varying degrees, and different chemical reactions occurred. Titanium carbide with minor vanadium alloying (Ti,V)C reaction products act as diffusion barriers when using PCD and cemented carbide, while the reaction products acting as diffusion barrier in pcBN is (Ti,V)B2. The presence of Mo and W in binder sites of pcBN reduces diffusional dissolution of cBN. Diffusion simulations agreed well with microscopy investigations and were enabled by the known temperature and pressure conditions of the static diffusion couples.

Non-metallic inclusions offer one of the most effective routes for improving the machinability of steels. However, the wear-reducing mechanisms activated by such inclusions are not fully understood. The interactions are notoriously difficult to predict due to the wide variety of steel grades, cutting conditions, and tool materials employed in industry. The interaction between PVD (Ti,Al)N coated cemented carbide tools, non-metallic inclusions, atmospheric oxygen, and the stainless steel 316L in a turning operation is therefore investigated here as a case study. The study includes turning experiments, nanometer resolution microscopy, and thermodynamic calculations. The paper explains how not only too high a contact pressures hinder the formation of protective deposits at the tool edge, but also how too low a contact pressure leads to excessive wear. A range of conditions specified in this paper must therefore be met for the two observed protective non-metallic inclusions Mg1Al2O4 and Al2Ca2Si1O7 to be preferentially deposited on a tool. Hence the coating wear is experimentally investigated, explained, and a thermodynamic calculation method for predicting the protective or degenerative potential of a deposit on the coating is presented.

The aluminum oxide-coating on cemented carbide tools used for metal cutting have been regarded as inert during cutting of steels. Because diffusional dissolution is not possible. Chemical degradation of aluminum oxide coatings is often overlooked, especially in the presence of ambient oxygen and non-metallic inclusions. High-pressure diffusion couples, advanced microscopy, and thermodynamics are used to investigate and predict the chemical degradation of aluminum oxide-coated tools. During interactions with steel and different combinations of inclusions with and without ambient oxygen. The results show that alumina is resistant to chemical degradation by steel in the absence of oxygen. However, this is not the case when oxygen and non-metallic inclusions are present. These experiments and microscopy together with the thermodynamic calculations allow for the creation of a method and guidelines for chemical wear modeling and steel inclusion engineering when machining with aluminum oxide-coated tools.

The ability to control the shape, distribution and composition of non-metallic inclusions has had an important impact on many aspects of steel making. One such impact is on the machinability. Ca-treatments have shown to be able to reduce the abrasiveness of oxide inclusions, improve chip-breaking and lead to formation of deposits that reduce tool wear. However, machining Ca-treated steels with Al2O3 coated cemented carbide tools has not been as advantageous as expected. This study investigates the mechanisms behind the anomalous wear of Al2O3 coatings when turning soft Ca-treated steels. Longitudinal turning tests at a range of speeds (vc = 100-600 m/min) show rapid localized degradation of the Al2O3 coating limited to the sliding zone. Detailed analysis of the degradation mechanisms was performed using scanning and transmission electron microscopy. The results demonstrate a presence of chemical interactions between the alumina coating and non-metallic inclusions. The interaction resulted in the formation of mainly calcium aluminates and partly alumina-magnesia spinel. In-operando infrared thermography measurements indicate cutting temperatures of 850-1000 degrees C. Thermodynamic calculations give that CaO and MgO readily reacts with Al2O3, while the reaction with CaS requires presence of additional oxygen at these cutting conditions. Additional turning experiments investigate the influence of oxygen by controlling the cutting environment by adding oxygen (compressed air) or removing oxygen (supply argon). These additional tests show that the presence of additional oxygen has a limited impact on the possible Ca-Al2O3 interaction. This demonstrat a potential for further machinability improvements by controlling the chemical interaction between Ca and Mg based non-metallic inclusions and alumina coatings.

Chemical, oxidational and diffusional interactions between the tool, chip and cutting environment are known tool wear mechanisms in machining. However, the interaction between tool, coating, workpiece, coolant and atmospheric oxygen can, under favorable conditions, lead to formation of reaction products that retard tool wear. A method with the ability to predict theses interactions, would therefore enable a better control over tool life in machining. An attempt to create such a modelling framework is developed in this study. This method can predict the phase composition and the driving force for degradation and the formation of protective interaction products in the cutting zone. This modeling approach is applicable across cutting processes in which chemical, diffusional and oxidational wear are dominant or present. This framework has been applied to investigate the interactions occurring in the cutting zone during turning of a medium alloyed low-carbon steel (Hybrid Steel® 55). A range of degradation events are predicted, as well as the formation of a protective corundum (Al,Fe,Cr)2O3 or spinel (Al,Fe,Cr)3O4 film due to an interaction between the Al-alloyed steel and the environment. Validation of the modeling was performed by studying tool wear and reaction products formed when machining with ceramics, PcBN and coated carbide tooling. Inserts are studied by the use of scanning and transmission electron microscopy, after cutting tests were performed. Additional tests were performed in different environments (dry, argon and coolant). The results confirmed the model predictions of oxidation and diffusion wear as well as the formation of an (Al,Fe,Cr)3O4 tool protection layer. Thus, the proposed thermodynamic framework seem promising to serve as a predictive instrument for the correct pairing of existing tool and workpiece combinations and cutting parameters, or for tailoring respective material compositions for intentional formation of a tool protection layer. As well as guidance on how to apply present and future kinetic models when concurrent interaction mechanisms are present. Which lead to a reduction and minimization of costly experimental machining tests.

Recently, PcBN tooling have been successfully introduced in machining Ni-based superalloys, yet our knowledge of involved wear mechanisms remains limited. In this study, an in-depth investigation of PcBN tool degradation and related wear mechanisms when machining Inconel 718 was performed. Diffusional dissolution of cBN is an active wear mechanism. At high cutting speed oxidation of cBN becomes equally important. Apart from degradation, tool protection phenomena were also discovered. Oxidation of Inconel 718 resulted in formation of γ-Al2O3 and (Al,Cr,Ti)3O4 spinel that were deposited on the tool rake. Also on the rake, formation of (Ti,Nb,Cr)N takes place due to cBN-workpiece interaction. This creates a sandwich tool protection layer forming continuously as tool wear progresses. Such in operando protection enabled counterbalancing tool wear mechanisms and achieved high performance of PcBN in machining.

During a metal cutting process, chemical wear can become the dominant mechanism of tool degradation under the high temperatures and contact pressures that arise between the tool and the metal workpiece. This study focuses on the chemical and diffusional interactions between superalloy Inconel 718 and cubic boron nitride (cBN) tool material with and without TiC binder. It covers thermodynamic modeling and experimental tests in the pressure range of 0.1 Pa to 2.5 GPa at temperatures up to 1600 degrees C. The methods used include diffusion couples under both vacuum and high pressure, transmission electron microscopy (TEM) analysis and in-situ synchrotron observations. It is shown that cBN is prone to diffusional dissolution in the metal and to reactions with niobium, molybdenum, and chromium from Inconel 718. Adding TiC binder changes the overall degradation process because it is less susceptible to these interaction mechanisms.